This invention relates to a device and a method for filtering a weight indicative electric signal (hereinunder referred to as "weight signal") to remove undesirable components therefrom. More specifically, this invention relates to a device and a method for enabling removal of oscillatory components attributable to external forced vibration from the ground or floor in addition to conventional undampered inherent oscillatory components attributitable to the structure of the weigher.
The weight signal produced from a weighing device such as a strain gauge load cell or force balance generally includes various undesirable components attributable to inherent vibration of the device, loading shock and the like in addition to a d.c. component indicative of the true weight of product to be weighed. The frequency characteristic of such weigher has a shape as shown in FIG. 1(a) and a peak frequency indicative of a feature of the inherent vibration is a frequency band 1 the in unloaded condition, while it is shifted to a lower frequency band 2 in loaded condition. In order to remove these undesirable components, therefore, three frequency ranges a, b and c may be provided as shown with reference to this frequency distribution to filter the weight signal by a filtering device having a frequency-versus-attenuation characteristic as shown in FIG.1(b). A skillful filtering system which enables this proceeding has been disclosed in the Japanese patent opening gazette No. 62-280625 and the Australian patent No. 578,181 of the same applicant as this application and the U.S. Pat. No. 4,817,026 assigned to this applicant. In this system, as shown in FIG. 2, an analog weight signal from a weigher 3 is amplified by an amplifier 4, filtered by an analog filter 5 and then supplied to a sample-and-hold circuit 6 to be sampled therein at a predetermined sampling frequency. A series of sampled weight signal output are digitized by an analog-to-digital (A/D) converter 7 and supplied to an arithmetic circuit 8. The analog filter 5 is adapted to previously remove noises relating to the sampling frequency (above a half of the sampling frequency). The following operation is applied to a series of input digital weight values D.sub.1, D.sub.2, D.sub.3 , . . . in the arithmetic circuit 8.
As shown in FIG. 3, the successive weight values D.sub.1 to D.sub.n1 (D.sub.1 to D.sub.10 in FIG. 3) are averaged and the resultant average is indicated as .sub.1 M.sub.1. Next, the successive weight values D.sub.2 to D.sub.n1+1 (D.sub.2 to D.sub.11 in FIG. 3) are averaged and the resultant average is indicated as .sub.1 M.sub.2. A similar averaging operation is repeated to obtain a time-series of averages .sub.1 M.sub.1, .sub.1 M.sub.2, .sub.1 M.sub.3 , . . . as shown in FIG. 3. Such series of averages are referred to as "moving (or progressive) averages of the first order" and the number n.sub.1 (10 in FIG. 3) is referred to as their "averaging number". Next, the successive averages .sub.1 M.sub.1 to .sub.1 Mn.sub.2 (.sub.1 M.sub.1 to .sub.1 M.sub.11 in FIG. 3), .sub.1 M.sub.2 to .sub.1 M.sub.n2+1 (.sub.1 M.sub.2 to .sub.1 M.sub.12 in FIG. 3) and so on are sequentially averaged to obtain a second time-series of averages .sub.2 M.sub.1, .sub.2 M.sub.2, .sub.2 M.sub.3 , . . . as shown. This series of averages are referred to as "moving (or progressive) averages of the second order" and the number n.sub.2 (11 in FIG. 3) is referred to as their "averaging number". By repeating a similar operation, moving averages .sub.r M.sub.i (i=1,2,3 , . . .) of r-th order having the averaging number n.sub.r can be calculated successively. Such operation is referred to as "multiplexing of moving (or progressive) averages".
As well known in the field of digital filters, the transfer function of the moving averages of the first order is expressed by the following equation. ##EQU1##
The former part of this equation is representative of the amplitude characteristic and the latter exponential part thereof is representative of the phase delay. In the equation (1), T denotes the sampling interval or period and .omega. equals 2.pi.f, where f denotes the oscillation frequency. The amplitude characteristic is therefore rewritten as follows, ##EQU2## where .phi.=n.sub.1 .pi.Tf. Accordingly, the amplitude becomes zero and when .phi. equals N.pi., where N is a positive integer excluding zero. Therefore, if f.sub.n1 =1/n.sub.1 T, the zero amplitude occurs at frequencies Nf.sub.n1. This feature is shown in FIG. 4 where the averaging number is assumed as ten (10). The frequencies Nf.sub.n1 (N=1,2 , . . . ) are referred to as "notch frequencies". Thus, it is understood that the moving average operation executed by the arithmetic circuit 8 functions as a filter having an attenuation band above about a half of frequency f.sub.n1 and notch frequencies representative of the zero amplitude appearing at integral multiples of f.sub.n1.
Similarly, the amplitude characteristic of the moving averages of the third order is expressed by the following equation. ##EQU3## This characteristic is shown in FIG. 5 where n.sub.1 =10, n.sub.2 =11 and n.sub.3 =12. In the drawing, the three notches of fn.sub.3, f.sub.n2 and f.sub.n1 exhibiting attenuation above 70 dB correspond to the frequency range b of FIGS. 1(a) and (b) and the range above 20 Hz exhibiting attenuation of at least about 40 dB corresponds to the frequency range c. The attenuation degrades with decrease of the frequency in the transition range below 16Hz corresponding to the frequency range a. As described above, the resonance components of inherent vibration of the weigher and various undesirable components attributable to shock and the like including frequency components above the intermediate portion of the range a can be filtered out substantially completely by suitably selecting the order number r and the averaging number n.sub.1, n.sub.2 , . . . in accordance with the above-cited prior art system.
The object of removal in the prior art is the inherent oscillatory components within the frequency range of about 20 Hz to 50 Hz which is attributable to the structure of the weighing device itself, while this frequency range was thereafter found to be better revised as 15 Hz to 50 Hz. However, it has been found that, in a highly accurate and sensitive weighing device, especially, in case of using a weighing unit having a relatively large tare mass, some of the oscillatory components attributable to external forced vibration acting on the device from the ground or floor have very low frequencies, such as 3 to 8 Hz. Such components may be insufficiently filtered which causes a problem when they are too much. While the strength of such oscillatory components of very low frequency necessarily varies with the external condition, addition of a separate filter effective to this frequency range will result in a problem of degradation of signal response and consequent reduction of weighing power of the weigher.
Accordingly, an object of this invention is to provide improved device and method for filtering a weight signal, which enables to change the attenuation in accordance with the external condition to remove the above-mentioned very low frequency components in the frequency region a of FIG. 1(b) as maintaining the frequency band having the attenuation characteristic of the prior art as shown in the same drawing.